Nicely stated. Einstein himself never said this was proof the either did not exist, only that light was independent of it. [I think I'm right on this.]Our problem with understanding light is that the things we really experience in our macro world that are waves need a medium to travel through. But, when we tried to find how fast the earth is traveling through the presumed medium for light (dubbed the "ether") we found that we could not measure and difference in any direction, even comparing measurements in different seasons, when we were in opposite positions in our orbit around the sun.
Interestingly, the modified Tychonic model is the one geocentric model that would work.So, rather than assume we were somehow in the middle of the universe with the sun revolving round us, we came up with the Special Relativity Theory.
It seems clear we don't understand it's weird behavior, but are you suggesting we aren't good at predicting how light will behave? [I won't be surprised if you're right, but I'm curious about this.]Getting unexpected, and unexplainable results from experiments with light are just a confirmation that we are not good at predicting what light can actually do, because we do not have a full understanding of how it works.
Matter and energy are not conserved but mass is. When matter is converted to energy, the energy has the same mass as the matter had. So, yes, light beams would attract each other. The practical problem is that a tiny bit of matter turns into an enormous amount of energy. The fireball from a 1 megaton atomic bomb represents only 1 kilogram of matter being converted into energy. Two light beams would hardly interact since their mass density is so low.
This Is Why Our Universe Didn’t Collapse Into A Black Hole | by Ethan Siegel | Starts With A Bang! | Medium
This explains it very well. The equations for a black hole are for a static assembly of mass, not a hot expanding mass. There was enough energy in the expanding fireball to stay ahead of its gravity and avoid collapsing.
Our physics breaks down before 10^-43 second which is why we can't explain what happened earlier. That does not mean the Big Bang didn't happen.
Inflation is required to explain current observations, that we can't explain what caused inflation does not mean it did not happen.
We see effects consistent with the presence of Dark Matter so we assume it is there even though we can't see it. We are simply stairstepping towards a better explanation.